Apparatus for adjusting torque and speed of a dentist handpiece D.C. motor

ABSTRACT

A D.C. motor control circuit for a dentist&#39;s hand drill including a feedback loop to vary the torque of the motor according to the load on the motor while keeping the motor speed constant. Power to the motor is provided through a three terminal regulating IC circuit. The IC regulating circuit is protected against power surges by an overload detecting circuit incorporated into the feedback loop.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for adjusting the torque of a D.C. motor running at constant speed.

2. Background of Related Art

The speed of a conventional dentist's hand drill motor is manually adjusted by adjusting the amount of electrical current controlled by a foot pedal regulator which varies the amount of resistance in the circuit. Under such circumstances the torque of the motor is variable and when the load on the motor increases, the speed of the motor decreases and finally stops.

SUMMARY OF THE INVENTION

Briefly described, the present invention comprises a motor for a dentist's hand drill which can operate at constant torque at any given speed of the motor. A three terminal regulator consisting of an integrated circuit IC chip, controls current from a D.C. source to the motor. A feedback loop provides a signal to an R.C. circuit connected to the common input of the three terminal regulator. The circuit further includes a surge protecting safety circuit for preventing overloads on the common input terminal of the IC.

The invention can be more fully understood by reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of the preferred embodiment of the present invention.

FIG. 2 is a circuit diagram of the safety circuit which protects the three terminal regulating IC.

DETAILED DESCRIPTION OF THE INVENTION

The same numbers will be used to refer to the same element as found in the figures which illustrate the invention.

In FIG. 1, electrical current from the A.C. electrical source 2 is rectified by a diode bridge, including diodes D₁,D₂,D₃ and D₄, and smoothed by capacitor C₁ into a D.C. output. The current from the D.C. circuit flows through resistor R₁ and the collector and emitter of transistor TR₁. From there current flows through the parallel connection of base protecting resistor R₂ and diode D₇.

Power to the motor M is regulated by a three terminal regulating circuit IC. Resistor R₃ controls the amount of feedback from the motor M. Selective switches R₅ and R₆ control the amount of feedback. A double pole single throw alternating switch SW controls which resistor R₅ or R₆ is chosen. The output terminal of the three terminal regulating circuit IC is connected to the base of transistor TR₁ and to the amplifier control diode D₈ and through resistor R₄ to the base of transistor TR₂. The emitter of transistor TR₂ is connected back to the output terminal of the regulating IC. The collector of transistor TR₂ is connected to the common COM terminal of the IC. The collector of transistor TR₂ is also connected to an RC time constant control capacitor C₄. The safety overload circuit shown in FIG. 2 is connected across terminals 3 and 4 during normal operation. The RC control capacitor C₄ is connected through resistor R₇ to diode D₁₀ and variable resistor VR₁ which controls the variable speed of motor M. Resistor R₇ is also connected to diode D₁₁ which is attached to the foot pedal switch of the variable resistor VR₂ and through resistor R₉ to the negative voltage D.C. source through diode D₆, capacitor C₃ and resistor R₈. The plus and minus outputs of the smooth D.C. source connect to a filter capacitor C₅ and blocking diode D₁₂ which are connected in parallel. The minus output terminal is connected to Zener diode D₁₃ and to variable resistor VR₁ in series with Zener diode D₁₃, resistor R₈ and diode D₆ which in turn connect the diode D₅ and capacitor C₂ which connect to diode D₂ and diode D₄.

As shown in FIG. 1 the D.C. current of the motor M includes a first D.C. path which flows through resistor R₁ transistor TR₁ and the base current resistor R₂. A second D.C. current path flows through the three terminal regulating IC and resistor R₃. The purpose of diode D₇ of the first D.C. current path is to bias resistor R₂ and prevent an overcurrent situation.

According to the circuit diagram of FIG. 1, an increase of current on the motor M causes a voltage increase across resistor R₃. This causes increased base current to transistor TR₂. The increased current flows through resistor R₅ or R₆ as selected by switch SW. The base current to transistor TR₂ increases the amount of the current to the common COM terminal of the regulating IC causing capacitor C₄ to charge. The increased feedback current to the COM terminal of the IC increases the current to the motor and accelerates the motor speed.

As the motor load increases, the motor current increases through the resistor R₃. The voltage increase across R₃ increases due to the increased actuating current to transistor TR₂ which controls the feedback current in accordance with the RC time constant of C₄ and R₅ or C₄ and R₆. The control of the capacitance discharge or capacitor C₄ by adjusting the manual variable resistor VR₁ through resistor R₇ and diode D₁₀ enables the setting of the speed of the motor M. And the control of the capacitance discharge of capacitor C₄ by adjusting the footsteps variable resisting VR₂ through diode D₁₁ and resistor R₉ causes an increase of the voltage to the common COM terminal of the IC which accordingly increases the output voltage of the IC.

In the meantime, the minus voltage rectifying circuit including capacitors C₂ and C₃ and diodes D₅ and D₆ are able to keep the capacitor C₄ voltage to zero so that the motor torque can be controlled at its lower speed range.

The amplification of transistor TR₂ is controlled by diode D₈ and the resistor R₄ connected between the emitter and the base of the transistor TR₂.

If a short should occur on the motor side of the circuit, damage to the IC is prevented by diode D₉ which is connected between the IC and the motor M and causes the capacitor C₄ to discharge its charge to the motor side instead of into the IC.

In order to protect the three terminal regulating IC from damage due to motor overcurrent or a motor short, an overload protecting safety circuit, as shown in FIG. 2 is connected between terminals 3 and 4. The safety switching circuit consists of transistors TR₃, TR₄, TR₅ and TR₆ which are connected between the common COM terminal of the IC and the capacitor C₄. This also places the safety circuit of FIG. 2 in connection with the collector of transistor TR₂. The safety circuit of FIG. 2 limits the overcurrent provided to the COM terminal of the IC by switching the path between terminals 3 and 4 on or off in response to the sensing of the voltage difference across both ends of resistor R₁.

As shown in FIG. 2, the terminal 1 connected between resistor R₁ and the transistor TR₁ is connected, in sequence, with the resistance R₁₆ diode D₁₅, diode D₁₆, diode D₁₇ and the base terminal of transistor TR₃ in series. Between diode D₁₅ and diode D₁₆, a second R.C. time circuit is formed by capacitance C₆ and resistor R₁₀ and also between the diode D₁₇ and the base terminal of transistor TR₃, a third R.C. time constant circuit is comprised of capacitor C₇ and resistor R₁₁. The collector of transistor TR₃ is connected to the base of transistor TR₄ which forms a switching circuit with transistors TR₅ and TR₆.

Resistor R₁₃ is connected to a point between the collector of transistor TR₄ and the base of TR₃. The collector of TR₆ is connected to the COM terminal of the IC and the emitter of the TR₆ is connected to a point between the capacitor C₄ and the collector of transistor TR₂.

As also shown in FIG. 2, the terminal 2 is connected to the power source and terminal 5 is connected to ground or earth. Diode D₁₄ permits the capacitor C₆ to be a temporary power source if the original power source suddenly cuts off. Resistor R₁₂ is connected to the base of transistor TR₄. Resistors R₁₄ and R₁₅ are respectively connected to the collectors of transistors TR₄ and TR₅.

The current from resistor R₁₂ actuates the switching circuit which comprises transistors TR₄, TR₅ and TR₆. Current from resistors R₁₂ normally turns the three transistors "ON" in the normal condition, this is the case because transistor TR₃ is normally in the "OFF" state because the voltage caused by the feedback resistors R₁₁ and R₁₃ do not provide sufficient bias to turn the transistor TR₃ on.

However, if any overload should occur on the motor M or a surge of current suddenly is drawn through the motor M as in the case of a short, an increased potential difference will arise across both terminals of the resistor R₁. The increased potential difference causes the capacitors C₆ and C₇ to discharge through resistors R₁₀ and R₁₁ according to their respective RC time constant. The current discharged by the capacitors C₆ and C₇ actuates the transistor TR₃ which had been in the "OFF" state. Transistor TR₃ then begins to conduct, i.e., thus turns "ON" and current from resistor R₁₂ flows through the collector the emitter of transistor TR₃. The conduction of transistor TR₃ shunts the current through resistor R₁₂ thereby robbing transistor TR₄ of its exitation. Transistor TR₄ then turns "OFF" which, in sequence, cause transistors TR₅ and TR₆ to turn off. Turning off transistor TR₆ cuts off the current to the COM terminal of the three terminal regulating IC. This causes an open circuit between C₄ and the COM terminal of the IC thereby preventing electrical damage to the IC.

In view of the foregoing, it is clear that the invention comprises a D.C. motor control circuit for adjusting the torque of a D.C. motor at a constant speed. In addition, the invention comprehends a means for preventing damage to the regulating integrated circuit IC if there should be a motor overload or short condition.

The invention has been described with reference to the preferred embodiment thereof it will be appreciated by those of those of ordinary skill in the art that various changes can be made to the parts and the circuit just described without departing from the spirit and scope of the invention as a whole. 

We claim:
 1. An adjustable torque D.C. motor control circuit apparatus for controlling the torque of small medical motors at a constant speed, said apparatus comprising:a source of positive and negative D.C. potential; a three terminal regulating circuit connected to said source of D.C. potential and having one input connected to said D.C. source, an output connected to said motor and a common terminal; a first capacitor and resistor means connected between said common terminal and said source of D.C. potential; feedback means connected between said motor and said first capacitor and resistor means for sensing the increased current load on said motor and for increasing the voltage to said common terminal of said three terminal regulating circuit to increase the output voltage of said IC circuit; first variable resistor means connected to said first capacitor and resistor means for controlling the discharge of said first capacitor and resistor means and thereby controlling the speed of said motor; a second variable resistor means also connected to said first resistor and capacitor means for controlling the speed of said motor, said second variable resistor means comprising a foot step control of variable resistor; IC protection means connected between said first capacitor and resistor means and the common terminal of said IC circuit for protecting said IC circuit from a sudden surge of current if an overload occurs on said motor; a load resistor R₁ connected across the input and output terminals of said IC regulating circuit; a second capacitor and resistance means connected across said load resistance R₁ ; transistor switch means connected to said second capacitor and resistance means, the output of said transistor switch means being connected between said common terminal of said IC circuit and said first capacitor and resistance means, wherein a change of potential across said load resistor R₁ above a certain predetermined value will cause said transistor switching means to electrically disconnect the common terminal of said IC regulating circuit to said first capacitor resistance means. 